From 69d9a2d50ef30b3204702bd2b47f4db34d927ac1 Mon Sep 17 00:00:00 2001
From: ZZiigguurraatt <187441685+ZZiigguurraatt@users.noreply.github.com>
Date: Mon, 29 Jun 2026 14:36:42 -0400
Subject: [PATCH] collectors: fix nil Logger panic in collector startup

NewChannelsCollector launched its cache-refresh goroutine at
construction time, but the package-global Logger was only initialized
later in PrometheusExporter.Start via initLogRotator. If
refreshClosedChannelsCache failed before Start ran, the goroutine
dereferenced a nil Logger and panicked.

Fix the initialization ordering: move initLogRotator to the top of
NewPrometheusExporter so logging is ready before any collector is
constructed and before any collector goroutine can run. The constructor
now returns an error; a logging init failure is fatal and surfaces on
stderr so it stays detectable instead of being silently swallowed.

Fixes #129.
---
 collectors/channels_collector.go |  5 ++++-
 collectors/prometheus.go         | 25 ++++++++++++++-----------
 lndmon.go                        |  5 ++++-
 3 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/collectors/channels_collector.go b/collectors/channels_collector.go
index db56032..056fa5c 100644
--- a/collectors/channels_collector.go
+++ b/collectors/channels_collector.go
@@ -194,7 +194,10 @@ func NewChannelsCollector(lnd lndclient.LightningClient, errChan chan<- error,
 		quit:                quitChan,
 	}
 
-	// Start a ticker to update the cache once per 10m
+	// Start a ticker to update the cache once per 10m. This is safe to
+	// launch here because NewPrometheusExporter initializes logging before
+	// constructing any collector, so the package-global Logger is ready by
+	// the time this goroutine can log on failure.
 	go func() {
 		ticker := time.NewTicker(cacheRefreshInterval)
 		defer ticker.Stop()
diff --git a/collectors/prometheus.go b/collectors/prometheus.go
index 42be6ad..c01fc61 100644
--- a/collectors/prometheus.go
+++ b/collectors/prometheus.go
@@ -94,7 +94,19 @@ func DefaultConfig() *PrometheusConfig {
 // the address to listen for Prometheus on and an lnd gRPC client.
 func NewPrometheusExporter(cfg *PrometheusConfig, lnd *lndclient.LndServices,
 	monitoringCfg *MonitoringConfig,
-	quitChan chan struct{}) *PrometheusExporter {
+	quitChan chan struct{}) (*PrometheusExporter, error) {
+
+	// Initialize logging before constructing any collectors. Some collectors
+	// launch background goroutines that log on failure, so the package-global
+	// Logger must be ready beforehand. A failure here is fatal: we return the
+	// error so lndmon exits instead of running with collectors that would
+	// dereference a nil Logger.
+	if err := initLogRotator(
+		filepath.Join(cfg.LogDir, defaultLogFilename),
+		defaultLogFileSize, defaultMaxLogFile,
+	); err != nil {
+		return nil, err
+	}
 
 	// We have six collectors and a htlc monitor running, so we buffer our
 	// error channel by 8 so that we do not need to consume all errors from
@@ -145,21 +157,12 @@ func NewPrometheusExporter(cfg *PrometheusConfig, lnd *lndclient.LndServices,
 		htlcMonitor:     htlcMonitor,
 		paymentsMonitor: paymentsMonitor,
 		errChan:         errChan,
-	}
+	}, nil
 }
 
 // Start registers all relevant metrics with the Prometheus library, then
 // launches the HTTP server that Prometheus will hit to scrape our metrics.
 func (p *PrometheusExporter) Start() error {
-	err := initLogRotator(
-		filepath.Join(p.cfg.LogDir, defaultLogFilename),
-		defaultLogFileSize,
-		defaultMaxLogFile,
-	)
-	if err != nil {
-		return err
-	}
-
 	Logger.Info("Starting Prometheus exporter...")
 	if p.lnd == nil {
 		return fmt.Errorf("cannot start PrometheusExporter without " +
diff --git a/lndmon.go b/lndmon.go
index 44e5bae..5e69a1a 100755
--- a/lndmon.go
+++ b/lndmon.go
@@ -78,9 +78,12 @@ func start() error {
 
 	// Start our Prometheus exporter. This exporter spawns a goroutine
 	// that pulls metrics from our lnd client on a set interval.
-	exporter := collectors.NewPrometheusExporter(
+	exporter, err := collectors.NewPrometheusExporter(
 		cfg.Prometheus, &lnd.LndServices, &monitoringCfg, quit,
 	)
+	if err != nil {
+		return err
+	}
 	if err := exporter.Start(); err != nil {
 		return err
 	}